Two serum proteins, C-reactive protein (CRP) and serum amyloid P component (SAP), are closely related in their primary structures. Perhaps the key difference between the two is their mode of biosynthesis; whereas SAP occurs in normal serum at constant levels regardless of the disease state, the serum level of CRP raises rapidly with the onset of inflammation. CRP is believed to mediate the removal of chromatin present in the cellular debris following cell death. The function of SAP is unknown. We found in the primary structure of SAP an amino acid sequence, YIGR, which resembles that of the cell attachment peptide. YIGR tested negative for cell attachment activity. In contrast to our expectations, a control peptide of 13 amino acids from the primary structure os SAP was found to have strong cell attachment activity. This SAP peptide was found to bind to fibronectin. Thus, we have identified a unique fibronectin-binding peptide that may be useful as a substrate for attachment of cells to insoluble matrices. This novel cell attachment peptide may be formed by proteolysis of native SAP in vivo. This may be a first step in the repair of damaged tissue. We have also found small peptides in the CRP structure that resembles tuftsin, Thr-Lys-Pro-Arg, and that have the ability to stimulate human B cell proliferation. Maintaining a supply of these biologically active peptides may be an important role for CRP.